The invention relates to a method of making a humidity sensor. The sensor comprises an electrically insulating substrate bearing an electrode system comprising a pair of noble metal electrodes. The electrodes are electrically connected by an electrically conductive, moisture-sensitive layer present on the substrate. The invention also relates to a humidity sensor made by such a method, to a hermetically sealed electronic module containing such a humidity sensor, and to a hermetically sealed surface acoustic wave device containing such a humidity sensor. Such an electronic module may for example, be a discrete semiconductor device, an integrated circuit or a computer module.
Humidity sensors for determining the moisture content of gaseous atmospheres are well-known. These sensors comprise a substrate bearing a pair of noble metal electrodes between which a layer of a hygroscopic agent, for example phosphorus pentoxide or lithium chloride, is disposed. The hygroscopic agent absorbs the moisture from the gaseous atmosphere. When the humidity sensor is used to determine the moisture content of a flowing gaseous atmosphere, the moisture content is usually determined coulombmetrically; a direct current voltage is applied across the electrodes, the water absorbed by the hygroscopic agent being electrolyzed, a steady state is then established between the moisture content of the gas and the moisture content absorbed in the hygroscopic agent.
United Kingdom Patent Specification No. 1,018,192 describes such a humidity sensor in which the hygroscopic agent is phosphorus pentoxide. A disadvantage of such sensors is that after the hygroscopic agent has been exposed to an atmosphere having a relatively high moisture content, for example the ambient atmosphere, it takes a considerable time to establish equilibrium at a relatively low moisture content. There will, of course, be limitations on the environment in which phosphorus pentoxide can be used as the hygroscopic agent. The humidity sensors described in Specification No. 1,018,192 are intended for use in flowing gaseous atmospheres.
An article entitled "The use of Relative Humidity Sensors to Monitor the Atmosphere within Hermatically Sealed Electronic Modules" by Elias J. Amdur and Harold G. Lofgren, (in Humidity and Moisture, Measurement and Control in Science and Industry, Volume 2, Applications edited by E. J. Amdur, Reinhold 1965), pages 428-432), describes the use of humidity sensors to monitor the humidity content of the atmosphere within computer modules. The moisture-sensitive elements in these sensors were polyvinyl alcohol-lithium chloride films, and had sensitivites over the range from 3.5 to 16% relative humidity. The moisture was determined by measuring the resistance of the moisture-sensitive elements using an A.C. ohmmeter.
It is well-known in semiconductor technology that all materials adsorb and absorb oxygen, nitrogen and water from the ambient atmosphere. Hermetically sealed housings are used to isolate semiconductor devices from the ambient atmosphere in which the device is used. The device may be sealed into such a housing in an inert atmosphere, for example dry nitrogen. Some semiconductor devices are affected by oxygen, and many semiconductor devices are affected by moisture. If the device is properly sealed into a housing, there will be no ingress of moisture into the housing containing the device after sealing. But moisture will be evolved into the atmosphere inside the sealed housing as a result of desorption from the inside surfaces of the housing and the surfaces of the device if the surfaces have not been dried adequately prior to sealing the housing.
Mass spectroscopy has been used to examine the moisture contents of the atmosphere within a sealed package, but this is a technique which cannot be used for continuous moisture monitoring. This technique is not suitable for measuring low moisture contents in atmospheres of packages having small internal volumes.